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Evidence continues to mount that physical exercise reduces a person’s risk of dementia.

In some of the latest research, scientists measure actual physical activity, rather than rely on people's imperfect memories.

Most researchers studying physical activity and dementia rely on self-reporting. So they'll ask people in a study about what exercise they've had in the past week, for example.

But there are problems with self-reporting. Laura Middleton of the Sunnybrook Research Institute and the University of Waterloo in Canada says for one thing, people just don't remember what they've done. Also, people tend to report certain types of activities more than others.

"[Self-reporting] does a very good job of capturing jogging or biking or tennis, but does a relatively poor job of capturing low-intensity activity like walking or daily chores, which may also be important to the risk of cognitive impairment."

To get around that problem, Middleton measured physical activity with an established technique that uses doubly labeled water, made from isotope variants of hydrogen and oxygen. Participants in the five-year study drank a small amount of this special water, and by measuring the isotope variants in their urine, their energy expenditure can be calculated.

"What we found was a strong relationship between activity energy expenditure and the risk of incident cognitive impairment, she says, "with those of higher activity energy expenditure had 90 percent reduced risk of incident cognitive impairment over the follow-up period compared to those with very low energy activity expenditure."

Laura Middleton and her colleagues describe their findings online in the Archives of Internal Medicine, published by the American Medical Association.

In the same issue, another paper - this one from French researchers led by Marie-Noël Vercambre of the Foundation of Public Health in Paris - studied the exercise-dementia link in a large group of women with cardiac risk factors such as obesity or diabetes. In this study, the women who got the equivalent of a brisk, half-hour walk every day had a lower risk of cognitive impairment.

Dr. Eric Larson of the Group Health Research Institute in Seattle, Washington, says the studies add to the evidence that physical activity can reduce the risk of cognitive decline.

"It's not obvious to people that exercise would make your brain healthier," he says. "And as each study does more detailed analyses of special groups or a different way of making the measurements, it just makes the scientific basis for this relationship a lot more convincing."

Larson writes that, with accumulating evidence of the link, research should now focus on how best to encourage people to be active, especially in later life.

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